Project Summary/Abstract (Molecular and Transgenic Core) The Molecular and Transgenic Core (Core A) of this Program Project grant develops and distributes the genetic tools and strategies to target and experimentally manipulate 5HT3aR cortical interneurons, a subpopulation of locally projecting neurons in the cerebral cortex that release GABA, an inhibitory neurotransmitter. For this renewal, the Core is particularly focused on devising and implementing intersectional genetic methods using transgenic mice and viral vectors to access the specific 5HT3aR interneuron subtypes that participate in the circuitry of cortical layer 1 (L1), the most superficial layer in the cortex where long range inputs from other cortical areas as well as neuromodulatory inputs from subcortical areas converge on pyramidal neuron dendrites. The three projects supported by this Core propose to study these L1 interneurons from their specification throughout their postnatal development into adulthood. In that respect, the Core?s work is essential for all three projects outlined in this application. To date, L1 cellular diversity has been enigmatic due to the paucity of specific markers and genetic tools that can distinguish between different cell type populations. The tools developed by the PPG Core will: 1) permit real-time observations of L1 interneuron cell types on deeper layer pyramidal cell dynamics and plasticity, 2) enable the suppression or stimulation of each cell type?s activity in brain slices and in vivo, and 3) facilitate analyses of L1 IN subtype properties and connectivity within the cortical circuitry. This Core will also serve as a resource for both the neuroscience and mouse genetics communities, as reagents produced will have broad utility for: 1) studying cortical IN and pyramidal neuron interactions, 2) observing neuronal activity in vivo in specific IN subtype populations, 3) experiments utilizing targeted optogenetics and activity-modifying proteins for the excitation or inhibition of specific neuronal populations, and 4) circuit analyses employing next-generation neuronal tracing methods to determine afferent and efferent connectivity. The novel analyses and experimental approaches devised by the Core will enable the three labs within this collaborative grant to achieve their respective scientific goals, and will greatly enhance other research programs in the broader neuroscience community.!